A virtual private network (VPN) refers to a private network set up on a public network. A common technology for setting up a VPN includes the layer 2 tunneling protocol (L2TP), Internet Protocol security (IPSec), the secure sockets layer (SSL), and the like.
With development of wireless communications technologies, transmission bandwidth of wireless communication continuously increases, and a VPN technology begins to be used in combination with the wireless communications technologies. As shown in FIG. 1, when an enterprise establishes a VPN, a branch router on which a wireless communications module, such as a third generation (3G) mobile communication or long term evolution (LTE) module, is configured is used on the enterprise's branch network. The branch router may access the Internet over a radio link provided by the wireless communications module, and set up the VPN with a headquarters router that is connected to a headquarters network in order to implement data communication between the branch network and the headquarters network. Subscriber identity module (SIM) cards of different telecommunications operators can be respectively inserted into multiple wireless communications modules configured on the branch router. In this way, the branch router can use radio links of different telecommunications operators simultaneously to communicate with the headquarters router, which increases transmission bandwidth between the branch router and the headquarters router.
In a scenario in which the branch router simultaneously uses multiple radio links, the branch router and the headquarters router balance load of the radio links according to bandwidth contracted with the telecommunications operators. When strength of signals received by the branch router over any radio link decreases, or a network standard of the radio link switches to a network standard of a relatively low rate, for example, switches from a 3G standard to a second generation (2G) mobile telecommunications standard, actual bandwidth of the radio link is less than the contracted bandwidth, and link quality deteriorates. In this case, if the branch router or the headquarters router still balances the load of the radio links according to the contracted bandwidth, data transmission efficiency is unsatisfied. For example, contracted bandwidth of a radio link 1 and contracted bandwidth of a radio link 2 of the branch router are both 2 megabit (Mbit)/second (s). When actual bandwidth of the radio link 1 is less than 2 Mbit/s because of signal strength, but actual bandwidth of the radio link 2 is still 2 Mbit/s, the branch router and the headquarters router still perform routing in a polling manner according to the contracted bandwidth, and load of the radio link 1 is the same as that of the radio link 2. In this case, a packet loss may occur on the radio link 1.